This invention relates to a vacuum or protective atmosphere heat treating furnace which permits very rapid cooling of a load in the hot zone.
Vacuum furnaces are well-known in the art. It is often desirable to heat treat metal parts, particularly, steel parts, in a vacuum. The vacuum provides protection for the parts, the surfaces of which may react with and be contaminated by atmospheric gases at high temperatures. The vacuum also protects electric heating elements in the furnace. Additionally, the use of a vacuum at high temperatures reduces heat losses and thus heating costs.
To obtain the desired properties of the metal, referred to herein as a load, it is often necessary to quench the load to rapidly reduce its temperature. When parts are heated in air this may be done by quenching in water, oil or molten salt.
Alternatively, when heated in a vacuum furnace, the load may be moved from a vacuum chamber to a separate chamber which holds the quenching medium. However, when a load of steel parts is heated above about 1200.degree. C., movement of the load from the hot zone of the furnace to a separate quenching zone can easily deform the parts.
Another technique is to quench the parts with a blast of cold inert gas introduced into a vacuum furnace at the end of the heating cycle. This, however, may result in insufficient quenching due to the necessity to cool parts of the furnace in addition to the load, such as the heating elements, insulation and other structural elements which comprise a hot zone of the furnace.
A technique for avoiding insufficient quenching in a vacuum furnace is to remove the load from the hot zone prior to gas quenching. This may be done by building a vacuum vessel with two interconnected chambers. The hot zone is located in the first chamber and the load is heated in this chamber. After heating, the load is moved into a second chamber adjacent to the first and the door between the two chambers is closed. The load is then gas quenched in this second chamber. Some alloys, however, required heating temperatures near the melting point of the metal, which significantly reduces the strength of the parts. The movement of the load from one chamber to another under these conditions may deform the metal parts. It is, therefore, desirable to provide a technique for rapidly gas quenching a load without first moving it out of the hot zone.
The rate of cooling obtained during a gas quench is a function of the pressure of the gas as well as its flow velocity. To avoid insufficient quenching, vacuum furnaces have been built which can withstand superatmospheric pressure. A typical vacuum furnace can operate at absolute pressure of up to two atmospheres to permit quenching the steel parts with a higher pressure of gas.
Vacuum heat treating furnaces have also been designed for operation at absolute pressures of from five to ten atmospheres to get even more rapid cooling without moving the load. Such furnaces may be able to harden a two-inch steel part, as compared with hardening a one-inch steel part in a furnace that operates at pressures up to two bar absolute.
A vacuum furnace can readily be built to withstand a pressure of two atmospheres; that is, one atmosphere pressure above ambient atmospheric pressure. Such a furnace is often referred to as a two-bar furnace. The same construction techniques may be used for a two-bar furnace, as for a vacuum furnace that is not repressurized above atmospheric pressure. However, if a furnace is to be built for an internal pressure higher than two bars, it must be constructed, inspected and certified under the ASME boiler codes, which can significantly increase the manufacturing cost of the vacuum vessel.
It is, therefore, desirable to provide a vacuum heat treating furnace with a cooling rate greater than a two-bar furnace without increasing the rating of the furnace shell above two bar.